Effect of prazosin on spontaneous sympathetic-cholinergic activity.

Abstract Tonic sympathetic-cholinergic electrodermal (sudomotor) activity was measured in intact anesthetized and unanesthetized decerebrate and decerebrate-spinalized cats. Prazosin (3–100 μg/kg i.v.) depressed spontaneous electrodermal activity in intact anesthetized cats in a dose-dependent fashion (ED 50 4.8 μg/kg). Prazosin's action was almost totally abolished by monoamine depletion with reserpine and a synthesis inhibitor. The α 2 -adrenoceptor blocker, yohimbine (0.5 mg/kg i.v.), also antagonized prazosin-induced sympatho-inhibition. Prazosin reduced tonic sudomotor activity in unanesthetized decerebrate cats in a dose-dependent fashion (ED 50 5.5 μg/kg i.v.), but was without effect in spinalized preparations. These results support the hypothesis that, in this system, prazosin produces sympatho-inhibition indirectly by means of an α 2 -adrenoceptor-mediated mechanism, an effect which appears to be prejunctional. It is proposed that prazosin acts at the level of the spinal cord to facilitate ongoing α 2 -adrenergic inhibition arising from supra-spinal loci.